The present invention relates to the control of stepless transmissions for use in vehicles, or the like. More particularly, the invention relates to a control device for a stepless transmission arrangement in which an improved valve structure is employed in the hydraulic control system thereof.
U.S. Pat. No. 4,152,947, granted May 8, 1979 to van Deursen et al. discloses a stepless transmission arrangement in which an endless belt is wound between primary and secondary pulleys having movable discs and in which the transmission ratio is changed by altering the belt winding position according to the axial displacement of the respective movable discs. A hydraulic control system including hydraulic actuators for each of the movable discs is employed for controlling the disclosed transmission.
The described hydraulic control system includes a first control valve that controls the supply of hydraulic fluid to the movable disc on the secondary pulley so as to increase belt tension by movement of the disc in a way that decreases the transmission ratio. A second control valve controls the supply of hydraulic fluid to the movable disc in the primary pulley so as to decrease the transmission ratio upon an increase in primary speed and to increase the transmission ratio as primary speed is reduced.
The first control valve is an overflow valve having a movable sleeve exposed at one end to the dynamic pressure corresponding to the primary speed generated by one of the pulleys. The other end of the movable sleeve is subject to a pressing force imposed by a control spring supported by a movable spring bearing. Such movable spring bearing is connected to a detecting link that is in sliding contact with the outer periphery of the pulley and adapted to move in one direction weakening the pressing force of the control spring as the transmission ratio is reduced and in the reverse direction increasing the pressing force of the control spring as the transmission ratio is increased.
The second control valve in the described arrangement comprises a balance-type valve structure having a pilot sleeve on one end of which is imposed the dynamic pressure corresponding to the primary speed generated by the pulley and on the other end of which is imposed the pressing force of a control spring supported by a movable spring bearing. The movable spring bearing is connected to a control lever adapted to input an external signal and is adapted to move in a direction to increase the control spring pressing force upon an increase in primary speed and, conversely, in a direction weakening such pressing force upon a reduction in primary speed.
In the described stepless transmission control system the spring bearing for the control spring urging the movable sleeve of the first control valve is directly connected to the detecting link. Consequently, even when the system is in a balanced condition in which the output speed of the transmission is constant, the resilient force of the control spring continuously acts on the detecting link through the intermediary of the spring bearing. As a result of this resilient force, the detecting link and the outer periphery of the pulley are in continual contact with one another whereby, even in a balanced system, frictional forces are generated that contribute to the frictional resistance of the system.
An attempt at solving such problem is disclosed in Japanese Patent Application Publication No. 56-66553 wherein a counter spring is employed in the balanced condition to establish a counterforce in opposition to the action of the control spring on the detecting link. This measure is, however, not totally dispositive of the problem.
Another problem in the described prior art stepless transmission arrangement results from the fact that the spring bearing for the control spring urging the movable sleeve of the second control valve is adapted to be driven by a bellows operating in a pressure detecting portion of the system to introduce a negative throttle pressure as an external input. As a result of this, the stroke control of the pilot valve becomes indirect requiring a complicated structure for the actuator for inputting a speed signal. While the device described in the aforementioned Japanese Patent Application Publication No. 56-66553 adopts a direct control system and is, accordingly, simple in its structure, because the resilient force of the control spring acts on a rod for inputting the speed signal, a large operating force is required to produce this action whereby control performance is reduced.
It is to the amelioration of these problems, therefore, that the present invention is directed.